This application claims the benefit under 35 U.S.C. xc2xa7365 of International Application PCT/EP/EP00/06476, filed Jul. 7, 2000, which claims the benefit of German Application No. 19933135.0, filed Jul. 19, 1999.
The invention relates to an electrical insulation device for bidirectional connecting lines/bus lines with the use of optocouplers.
The invention is based on an electrical insulation device for bidirectional connecting lines of the generic type of the independent claims 1 and 2. If a plurality of independent apparatuses are interconnected via lines, then it is often necessary to provide electrical insulation between the interconnected components. This applies particularly whenever the connected apparatuses are installed e.g. in a manner distributed in a building. This is because in this situation relatively large potential differences between the apparatuses can occur, which are caused e.g. by different potentials on the power supply lines. Such potential differences may occur in the range from a few millivolts up to a number of volts. Potential differences of this type may be present with greater or lesser stability. They may vary e.g. in accordance with the instantaneous total power consumption in the building. However, they may also fail momentarily, with destructive effect, e.g. due to a lightning strike in the building itself or in the vicinity of the building.
In the less severe case, the data signals and/or control signals which run via the bus connections are merely corrupted. However, they can lead to the destruction of the connected circuit sections.
The problem of undesirable earth loops, caused by the connecting lines, frequently arises. By way of example, induced current can flow through the cable screen of the bus connection and likewise corrupt the transmitted data signals. If the induced potential difference is large enough, persons who happen to be handling the corresponding bus connection cable could also be injured.
Therefore, the requirement for complete electrical isolation of the stations which are connected to one another by lines is necessary.
One example of a bus system in which electrical isolation of the components which are connected to one another is required is the IEEE 1394 bus standard, which has recently acquired increasing importance. The exact designation of this bus standard reads as follows: IEEE Std 1394-1995, xe2x80x9cIEEE Standard for a high performance serial busxe2x80x9d of 12.12.1995.
What is involved is a bus system containing two data line pairs and also two power supply lines earth and Vcc and also a cable screen in the bus connection cable. The two data line pairs allow synchronous serial data transmission. What is probably one of the most outstanding properties of the bus system is that data transmission is possible at very high data rates of 100 megabits per second up to 400 megabits per second.
With regard to the realization of electrical isolation of stations which are connected to one another via the bus, two explicit circuit realizations are specified in Appendix J.6 of the abovementioned standard. In both cases, electrical isolation is performed between the data link layer module and the physical layer module. A transformer connected up appropriately with resistors and capacitors is used for the electrical isolation in one case, and capacitive decoupling is provided for the electrical isolation in the other case. These solutions have assumed, however, that the data link layer module and the physical layer module are present as separate chips. It has been shown in retrospect that the capacitive isolation of the two modules does not constitute a reliable solution in practice at the high frequencies. Instances of signal corruption and interfering irradiation have occurred. In the case of electrical insulation using a transformer, moreover, there is the disadvantage that this solution can no longer be used if the data link layer module and the physical layer module of the bus interface are intended to be integrated on a single chip.
Furthermore, it is known to use so-called optocouplers for the electrical isolation of circuit units which are connected to one another.
The object of the invention is to specify an electrical insulation device in particular for bidirectional connecting lines which operates reliably even at very high frequencies and can be integrated very easily on a chip.
The object is achieved by means of the features of the independent claims 1 and 2. In accordance with a first embodiment of the invention, the electrical insulation device for bidirectional connecting lines comprises two separate optocouplers per bidirectional connecting line and a control unit, which generates switching signals in a manner dependent on control signals output by one of the two circuit units, which switching signals, via corresponding switches, activate one of the two optocouplers and deactivate the other optocoupler, and thus allow transmission of signals via the connecting line in one direction. This solution does not require poorly integrable components such as transformers. Moreover, the aforementioned control unit can be constructed in a simple manner and can be readily integrated on a chip.
The second solution according to the invention in accordance with claim 2 manages with just one optocoupler per bidirectional connecting line. This is achieved by modifying the control unit in such a way that, in a manner dependent on the control signals output by one of the two circuit units which are connected to one another, the said control unit changes over the effective direction of the optocoupler with regard to the associated connecting line. In this case, it is necessary merely to provide two more switches for the switching operations. The implementation of such switches does not constitute a problem for the chip design. Therefore, this solution provides an electrical insulation device which can be integrated in a particularly simple manner.
Further advantageous developments and improvements of the devices mentioned in claims 1 and 2 are possible by virtue of the measures evinced in the dependent claims. In accordance with claim 4, the electrical insulation device may very advantageously be provided between a data link layer module and a physical layer module of a bus interface. If the solution is used in the case of an IEEE 1394 bus interface, then it suffices for the respective control unit to evaluate the control signals on the two control lines CTL [0:1] of the connecting bus between the two modules in order to activate the corresponding optocoupler or to change over the effective direction of the optocoupler.
Tristate drivers, in particular, may expediently be used as switches for the changeover between the optocouplers or for changing over the effective direction of the optocoupler, the said tristate drivers being driven correspondingly by the control unit.